1. Field of the Invention
The invention concerns a process for the continuous removal of undesirable components from solid particles, and is particularly directed to the removal of soluble components from microsphere material to be used for the production of nuclear fuel, the soluble components being removed using a solvent streaming through a bed of the microsphere material in counterflow. The invention also is directed to an apparatus for performing the process.
2. Description of the Prior Art
In nuclear reactor technology a particulate material containing thorium, uranium or plutonium is used in the manufacture of nuclear fuel. In this regard, fuel elements used in gas-cooled high-temperature reactors are formed of a graphite matrix in which fission or breeder materials in the form of coated microspheres are embedded. Also, the loading of fuel rods used in light water and breeder reactors can be carried out with a particle-shaped fuel (Spherepac-Fuel). The diameter of such particles is in general in the range between 50 .mu.m and 5 mm.
As a rule, such microspheres are produced by wet-chemical methods. In these processes the microspheres are cast, whereby a solution of suitable composition separates into drops and undergoes a chemical process resulting in the solidification of the drops. The solidified drops (microspheres) are as a rule of a gel-like consistency and thus not very resistant to mechanical stresses.
After solidification the microspheres will generally still contain by-products which have to be eliminated from the microspheres before they are further processed. This is achieved by dissolving the undesirable components in a suitable solvent.
The microspheres used for the production of nuclear fuel contain toxic, radioactive components, a part of which is carried over into the solvent in the course of the dissolution of undesirable components. The used solvent, however, can only be permitted to be released into the environment after special conditioning treatment and is therefore mostly recycled. Consequently, for economical and ecological reasons, solvents should be used as sparingly as possible, i.e., the solvent should be charged with foreign bodies to the maximum possible extent. For the treatment of such particles, systems involving continuous counterflow are used.
In known apparatuses for the continuous removal of soluble components of solid particles, the solid substance is carried in counterflow in a stream of solvent either by a device in the fill of solid material or by a conveying device for the solid material.
To the first group belong equipment using a screw conveyor (Hildebrandt Extractor). In such equipment, since the product of extraction is exposed to mechanical stresses caused by the movement of the screw, the purification of microspheres in such devices can cause damage to or even destruction of the product.
To the second group of equipment belong extractors fitted, among others, with a conveyor belt, a bucket elevator and a bucket-wheel. These extractors are mechanically complicated and are, as a rule, unsuitable for products that are too delicate for mechanical handling. Furthermore, in handling radioactive materials there are additionally decontamination problems which, for this kind of apparatus, are difficult to solve and are accordingly expensive.
In other known processes a solvent is forced through a solid bed made up of the particles to be purified. Experiments have shown that the pressure drop across the solid bed can also damage the mechanically delicate microspheres. In addition, as is known, in the case of solid beds there is a risk of channel-forming. In such cases, the solvent no longer flows uniformly through the pile or fill of microspheres but rather it flows through the individual channels in the bed of microspheres so that the particles are not sufficiently purified.
The object of the present invention is therefore to eliminate or reduce the drawbacks of the known processes and to provide a process and apparatus which can vouchsafe that undesirable components are substantially eliminated from mechanically delicate solid particles while maintaining their shape.